Most motors are designed to set amount of work, usually rated in either watts or horsepower, which is 746 watts per HP.
Watts law states that Watts = Volts x Amps. If a particular motor needs to do 1 horsepower of work at 120 Volts it will draw about 6.22 amps. And yes in an inductive load like a motor it's not quite as simple as VxA=P but we are keeping it simple here.
A motor designed to do the same amount of work (1HP) at 240v will draw half the Amps (3.11).
This does not make the second motor “more efficient” because the power company charges by the Kilowatt NOT by the amp.
) If you take a load that is designed for a particular voltage and you DROP the voltage it will also decrease the wattage according to Watts law (Watts = Volts x Amps) as well as decrease the amperage according to Ohm's law (so long as the resistance remains the same).
Let's say you take a 5KW heat strip that is rated as 5Kw at 240v and you instead connect it to 120v.
It would then only produce 1.25 kw and draw 1/4 the amps, this is because while we may call it a “5 Kilowatt heater” it is actually just a fixed resistor designed to do 5 kilowatts per hour of work in the form of heat at 240 Volts. Cut the Volts in half you also cut the amps in half and you decrease the amount of work done down to 1/4 because Watts = Volts x Amps.
Brake Horsepower (BHP), motor nameplate HP, and the actual electrical power (watts) a motor will consume are different, but related concepts. This gets confusing, so we will tackle them one at a time.
Let us start with simple definitions of work, power, and horsepower.
Work = distance in feet x force in pounds, both applied in the same direction.
Power = time rate of doing work, or work / time. Some units of measuring power are watts, kilowatts, HP, etc.
Horsepower (HP) is defined as doing work at the rate of 33,000 ft x lbs / minute. A horse can do work at this rate continuously without being overloaded. A human cannot sustain doing work at the rate of 1 HP for very long. There is an interesting video of an Olympic cyclist on a stationary bike generating enough power to operate a toaster at @ 700+ watts (746 watts = 1 HP).
He is exhausted after powering the toaster just a few minutes to toast 1 piece of bread. Put another way, imagine continuously lifting an endless supply of 200 lb barbells from the floor up to a height of 7 feet (clean & jerk). To do work at the rate of 1 HP, you would have to lift @ 24 of these barbells up 7 feet, every minute. Every minute, 24 barbells, 200 pounds each, lifted up 7 feet. 33,000 ft x lbs / minute / 7 feet x 200 lbs = 23.6 barbells / minute. This helps us understand why it takes approximately five people to do the work that one horse can do.
Motor nameplate HP indicates the amount of mechanical HP a motor can safely deliver via its shaft without overloading/overheating. Motors are not always delivering full nameplate HP to the load they are driving. The load required by fans, pumps, and compressors usually depends on the fan or pump’s design, the flow rate (cfm, gpm) it is handling, and the pressures that are developed. Thus, a motor with the capability of 3 HP may be driving a load that only requires 2 HP.
Brake horsepower (BHP) refers to the actual mechanical horsepower received at the shaft of the driven machine, such as a fan, pump, compressor, etc.
The watt is also a unit of power. 1 mechanical HP is equivalent to @ 746 watts. Electric motors do not operate at 100% efficiency, thus some of the wattage consumed by a motor converts directly to heat. The rest (the majority) of the wattage provides useful mechanical shaft power to the load that is being driven. For example, a 3 HP motor that is fully loaded and is operating at 90% efficiency will consume a total of 3 HP / 0.90 = 3.33 HP in electricity…or 3.33 HP x 746 watts / HP = 2,487 watts. Of the total 2,487 watts of power being consumed by the motor, only 3.0 x 746 watts / HP = 2,238 watts are converted to useful mechanical shaft power.
To avoid confusion, make sure you understand these different units, or ways of measuring & expressing power.
In Residential and light commercial HVAC we work primarily with PSC (Permanent split capacitor) motors. However, there are some other types that are good to be aware of.
PSC (Permanent Split Capacitor)
A common medium torque single phase motor with a run capacitor always in the circuit. This type makes up the majority of HVAC motors (condenser fans motors, blower motors compressors)
CSCR – (Capacitor Start, Capacitor Run)
A higher starting torque motor that uses a run capacitor as well as a start capacitor. The start capacitor is removed from the start circuit shortly after starting using a potential, current or centrifugal relay.
CSIR – (Capacitor Start, Induction Run)
These motors are fairly rare and utilize a start capacitor and no run capacitor.
Three phase motors require three phase power and do not require capacitors.
Shaded pole motors are very small, low torque motors. They can only run in one direction and they do not utilize capacitors.
D.C. (Direct Current)
D.C. Motors work on Direct Current and (generally) utilize brushes to transfer an electrical charge to the armature (rotor) of the motor.
ECM (Electronically Commutated Motor)
This type of motor is a high-efficiency DC (or three phase depending on how you look at it) motor that uses no brushes, a permanent magnet rotor and utilizes electronically switched DC power to turn the motor at various speeds.
Typical induction motors are slaves of the electrical cycle rate of the entering power (measured in hertz ).
Our power in the US makes one full rotation from positive electrical peak to negative peak 60 times per second or 60hz (50hz in many other countries)
This means that the generators at the power plant would have to run at 3600 RPM if they only had two poles of power 2 poles (60 cycles per second x 60 seconds per minute = 3600 rotations per minute) in reality, power plants generators can run at different speeds depending on the number of magnetic poles within the generator. This phenomenon is replicated in motor design.
The more “poles” you have in a motor the shorter the distance the motor needs to turn per cycle.
In a 2 pole motor it rotates all the way around every cycle, making the no-load speed of 2 pole motor in the US 3600 RPM.
A 4 pole motor only goes half the way around per cycle, this makes the no-load (Syncronous) RPM 1800
6 pole is 1200 no load (no slip)
8 pole is 900 no load (no slip)
So when you see a motor rated at 1075 RPM, it is a 6 pole motor with some allowance for load and slip.
An 825 RPM motor is an 8 pole motor with some allowance for slip.
A multi-tap / multi-speed single phase motor may have three or more “speed taps” on the motor. These taps just add additional winding resistance between run and common to increase the motor slip and slow the motor.
This means a 1075, 6 pole motor will run at 1075 RPM under rated load at high speed. Medium speed will have greater winding resistance than the high speed and therefore greater slip. Low speed will have a greater winding resistance than medium and have an even greater slip.
Variable speed ECM (Electronically commutated motor) are motors that are powered by a variable frequency. In essence the motor control takes the incoming electrical frequency and converts it to a new frequency (cycle rate) that no longer needs to be 60hz. This control over the actual frequency is what makes ECM motors so much more variable in ten speeds they can run.
So in summary. There are three way you can change a motor speed.
Change the # of poles (more = slower)
Increase slip to make it slower, decrease slip to bring it closer to synchronous speed